The present invention relates to the field of electromagnets, and more particularly to a solenoid in which the magnetic field is generated by conducting an electrical current through a coil made of a ceramic superconducting material.
A superconductor conducts an electric current with zero resistance (strictly speaking, true only for direct current). That is, the current flows through a superconductor without loss or resistive heating. The superconducting state of a superconductive material exists, however, only for temperatures, current densities, and magnetic fields below given critical values which are characteristic of that material. When these critical values are exceeded, the superconductive material becomes resistive (non-superconducting) to the flow of current.
The current-carrying capacity of superconductors can be quite high with critical current densities greater than 10.sup.6 A/cm.sup.2. Metallic superconducting wires can be formed into coils to form magnets that can produce magnetic fields many times that of the largest iron electromagnets, and they can do so in a package small enough to be held in the palm of one's hand. The superconducting wires of choice today for such high-field magnets use NbTi and Nb.sub.3 Sn as the conducting elements. These advanced materials are capable of carrying very large current densities that allow superconducting magnets to have magnetic fields in the range of 5-20 Tesla. A major drawback to metallic based superconductors, however, is that they must operate at a temperature of 4.2K. Such low operating temperatures generally require the use of liquid helium, an expensive and logistically difficult material to use.
In 1987, high temperature ceramic superconducting materials were discovered. In this field, high temperature refers to temperatures above the boiling point of liquid nitrogen (77K or -196.degree. C.); that is, the superconducting transition temperature, or critical temperature, T.sub.c, of these new materials is over 77K. Examples of such ceramic superconductors are YBa.sub.2 Cu.sub.3 O.sub.7 (T.sub.c =92K), Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.8 (T.sub.c =80K), and Bi.sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10 (T.sub.c =110K). Ceramic superconductors are very different than metallic superconductors, such as niobium (T.sub.c =9K), niobium-titanium (T.sub.c =17K), and niobium germanium (T.sub.c =23K), because ceramic superconductors are superconducting at much higher and more easily attained temperatures.
Ceramic superconductors have a unique morphology, resulting, in part, from the way they are synthesized and also from their intrinsically complex structure. A common method for preparing bulk pieces of ceramic superconductors is to press the powder of the superconductive compound into pellet form, which is then heated at high temperature so that adjoining pieces of powder in the pellet become connected by either a partial melting or diffusion process. Such a heat process is referred to as sintering. In sintered form, ceramic superconductors usually contain high quality grains of superconductor, which may carry large currents, separated by lower quality material referred to as weak links, or intergrain regions, which carry less current. Intergrain regions may consist of defective material (for example, off-stoichiometry or under-oxygenated), and material that includes misaligned connecting grains, impurities, or reaction by-products. Such material difficulties, along with the rather brittle nature of ceramics, have made ceramic superconductive materials to fabricate into useful shapes. For example, attempts to draw wire from ceramic superconductive materials capable of carrying large electric currents when operating in a superconducting state have resulted in only limited success. Ceramic wires suitable for winding a superconducting magnet are as yet unavailable.
Since before the turn of the century, magnets for motors, generators, and laboratory magnetic fields have been based on the electromagnet; In fact, the principle utilized in motors dates back to 1819, when the Danish physicist Hans Christian Oersted showed that electricity and magnetism were related. By 1821, the English scientist Michael Faraday had built a simple electric motor, laying the foundation for the development of practical electric motors and generators. In 1888, Nikola Tesla patented the alternating-current (AC) electric motor. A basic electromagnet operates as follows. A current-carrying wire, usually copper, is wound around a ferromagnetic material such as iron to form a solenoid or a torus. The current in the solenoidal windings produces a magnetic field intensity, H, inside and along the axis of the solenoid. The magnetic field, B, produced inside and along the axis of the solenoid is simply B=.mu..sub.r .mu..sub.o H, where .mu..sub.r is the relative magnetic permeability of the ferromagnetic material and .mu..sub.o is the permeability of free space. Thus, if the windings are wound around iron, for example, the large relative magnetic permeability, .mu..sub.r, of the iron in effect produces a large gain on the order of .mu..sub.r in the magnetic field produced in the iron by the current in the windings. For a good ferromagnetic material, .mu..sub.r may be on the order of several thousand for low magnetic fields. Thus, rather modest currents can produce large magnetic fields. This effect is limited, however, by the fact that the relative magnetic permeability saturates at a magnetic field corresponding to the complete alignment of all the magnetic domains in the ferromagnet. At this saturation field, one loses the large gain, and further increases in the current produce only very small increases in magnetic field. Thus, for all practical purposes, the magnetic field strength of an electromagnet is limited by the saturation field of its ferromagnet. For an electromagnet using an iron core, the limit of magnetic field strength is a little over 2 Tesla (20 kGauss). An electromagnet that produces such a magnetic field is usually water-cooled, requires a large power supply, and is very large and cumbersome.
With the discovery and development of metallic superconducting wire in the 1960's, first NbTi and later Nb.sub.3 Sn, magnet development took a quantum leap forward. A superconducting magnet does not require any ferromagnetic material in order to generate a large magnetic field and exhibits no hysteresis. Inside a long solenoid wound with superconducting wire at a density of n turns per meter, carrying a current, I, the supercurrent in the windings produces a magnetic field strength, H, inside the solenoid given by H=nI. Hence the magnetic field, B, inside the solenoid is given by B=.mu..sub.o H=.mu..sub.o nI. Practically speaking, the limit to the magnetic field, B, that the superconducting magnet can produce is determined by the critical current, I.sub.c, which is the maximum current the superconducting wires can carry and still remain superconducting. Below that limit, no heat is generated in the magnet; there are no electrical resistance losses. In fact, a superconducting magnet can operate in what is called the persistent mode. That is, once a current is established in the superconducting magnet, the power supply can be disconnected and removed. The field produced by the current will remain indefinitely as long as the superconducting magnet is kept cooled below its T.sub.c. With electromagnets, cooling water is needed to carry away the I.sup.2 R heat generated in the copper windings. However, the cooling requirements of superconducting magnets need only be enough to maintain the windings below the critical temperature, T.sub.c, of the superconducting winding material. Although a superconducting magnet derives no benefit from having a high-permeability core, the large currents that the superconducting wire can carry result in magnetic fields many times that attainable with electromagnets. The critical currents attainable from commercial superconducting wires can be used to generate magnetic fields from 5 to 20 Tesla, depending on the wire material and the design of the magnet, for commercial magnets cooled with liquid helium to a temperature of 4.2K. Thus, superconducting magnets produce maximum magnetic fields from 2.5 to 10 times the saturation field of iron. Superconducting magnets may also be small and light in weight (as for example, a few pounds). By contrast, a laboratory electromagnet usually weighs several thousand pounds.
Therefore, a need exists for a superconducting magnet which is capable of operating at temperatures which equal or exceed about 20K. A further need exists for a superconducting magnet which employs ceramic based superconductors as a superconducting medium.